The present invention pertains to the field of burners, particularly industrial burners of the type used in various high-temperature process applications. It is well established that, in burner systems used in industrial furnaces where two reactants are combusted, (i.e., where a hydrocarbon fuel is combusted with an oxidant) various nitrogen oxide compounds are generated (known collectively as NOx) which has been identified as an environmental pollutant. The reduction of NOx production has become the policy in recent years of various state and federal regulatory agencies. Typical mandates require NOx levels of about 30 ppmv for ambient temperature air. Thus, methods of NOx reduction are of great value.
The factors contributing to NOx production are understood, qualitatively if not quantitatively. In general, it is believed that NOx production is a path-dependent phenomenon resulting from uneven mixing of the fuel and oxidant, which results in sharp temperature gradients, localized peak flame temperatures and elevated oxygen concentrations in the hottest parts of the flame. Various techniques are typically used to reduce these factors. However, such schemes offer various tradeoffs in installed cost and operating efficiency.
A frequently used technique for reducing NOx emissions is external flue gas recirculation (FGR) in which inert combustion products are mixed with the oxidant and/or fuel streams upstream of the burner. This adds a thermal ballast to the system and reduces flame temperatures, thus inhibiting NOx formation. However, FGR systems require additional installation due to larger fans and motors and increased pipe requirements. FGR systems require more energy to operate and are less efficient in the yield of useful heat. Also, FGR components tend to have a short useful life, requiring increased maintenance and/or replacement expenses. During operation, FGR systems tend to be unstable and difficult to control, resulting in increased production expenses due to down time of the system. These difficulties are aggravated as lower NOx levels are attempted, and such systems may become economically unfeasible if further NOx reductions are mandated by the regulatory agencies.
Other burner system designs have been contemplated for complying with NOx production mandates that avoid the problems associated with external FGR. Such systems include air or fuel staged burners in which mixing of fuel and air takes place in multiple stages, allowing heat loss and dilution of reactants with products of combustion between the physically defined stages, thus reducing peak temperatures. However, staged burners are physically large and have complex oxidant and/or fuel passages, increasing installed costs and maintenance requirements.
Another method involves dilute reactant injection in which a furnace is heated to auto-ignition temperature, and fuel and oxidant are injected into the furnace in such a way that each entrain combustion products prior to mixing and combustion. While these systems provide very low NOx levels, additional penetrations to the furnace walls are required compared to conventional burners. This adds to the cost of a new furnace, and makes retrofitting an existing furnace difficult and expensive.